1. Field of the Invention
The present invention relates to a method for producing a glass preform for use in the production of an optical fiber. More particularly, it relates to a rod-in-tube method for producing a glass preform from which an optical fiber having improved strength and low attenuation of light is produced.
2. Description of the Prior Art
In a rod-in-tube method for producing a glass preform for use in the production of an optical fiber, a quartz glass rod which forms a core part of the glass preform and has a desired refractive index profile is preferably produced by the Vapor Phase Axial Deposition method (hereinafter referred to as the "VAD" method) and inserted coaxially into a glass tube of pure quartz. Then, an outer surface of the glass tube is heated by an oxyhydrogen flame to collapse the glass tube, and the glass tube and the glass rod are integrated so as to produce the glass preform for the optical fiber. Before the glass rod is inserted into the glass tube, an inner diameter of the glass tube and/or the diameter of the glass rod are preferably modified by heating, since the less the clearance is between the glass tube and the glass rod then, the less the eccentricity is of the core after the collapse.
When the clearance between the glass rod and the glass tube is extremely narrow, it is extremely difficult to prevent direct contact of the glass rod to the glass tube during insertion of the glass rod into the glass tube, which produces minute flaws on either or both of an inner surface of the glass tube and an surface of the glass rod. The flaws increase the attenuation of light and/or decrease significantly the strength of the resulting optical fiber. Particularly, when the flaws are large, the optical fiber is often broken in a subsequent drawing step of the glass preform, and a large diffusion loss of transmitted light occurs by the flawed part.